Stellar Survivor: Brown Dwarf Outlasts Red Giant

ESO
The lives of stars like the sun follow a predictable course: billions of years spent pumping out energy as hydrogen fuses into helium in a superhot core followed by an expansive phase when the fuel runs out. Such older sunlike stars become red giants, swelling to consume the hydrogen in the outer reaches of their atmospheres until that too is consumed and the swollen star's outer layer is expelled as a nebula. This leaves behind a small, so-called white dwarf consisting of the residual helium core. Now scientists have discovered an unusual binary star system consisting of a brown dwarf (a pseudostar 55 times the size of Jupiter but still too small to reliably fuse hydrogen) and a white dwarf. "Such a system must have had a very troubled history," says Pierre Maxted of Keele University in the United Kingdom who led the observations. "Its existence proves that the brown dwarf came out almost unaltered from an episode in which it was swallowed by a red giant."

The brown dwarf may have even gained mass from the common envelope that existed during the system's (known as WD 0137-349) red giant phase (depicted in the image above with the red center showing the future white dwarf and a black dot representing the brown dwarf). But it also lost some distance from its stellar neighbor. During its engulfment, the brown dwarf spiraled ever closer to the helium core until now the two orbit each other every 116 minutes; the brown dwarf travels at speeds of 800,000 kilometers per hour (nearly 500,000 miles per hour). Despite its name, the brown dwarf only survived thanks to its size. "Had the companion been less than 20 Jupiter masses, it would have evaporated during [the red giant] phase," Maxted notes.

But size will not save it for long in stellar terms; in roughly 1.4 billion years, the orbital period will decline to as little as one hour at which point the compact but powerful white dwarf will begin to suction material from the facing side of the brown dwarf. Such systems are known as cataclysmic variables and, as the white dwarf acquires new hydrogen, it will slowly acquire the mass for a spectacular nuclear chain reaction: a nova. So the brown dwarf may have survived the red giant but the white dwarf will consume it in a future thermonuclear explosion. The paper presenting the research appears in today's Nature.